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The effect of morphology on the electrochemical
properties of nanostructured metal oxide thin films : the
studies based on multi-scale time-resolved fast
electrogravimetric techniques
Fatemeh Razzaghi
To cite this version:
Fatemeh Razzaghi. The effect of morphology on the electrochemical properties of nanostructured metal oxide thin films : the studies based on multi-scale time-resolved fast electrogravimetric techniques. Chemical Physics [physics.chem-ph]. Université Pierre et Marie Curie - Paris VI, 2016. English. �NNT : 2016PA066346�. �tel-01477408�
I
Thèse de doctorat
Pour l’obtention du grade de Docteur
De l’Université Pierre et Marie Curie
École doctorale 388 - Chimie Physique et Chimie Analytique de Paris Centre
The Effect of Morphology on the Electrochemical
Properties of Nanostructured Metal Oxide Thin Films:
The Studies based on Multi-scale Time-resolved Fast
Electrogravimetric Techniques
Par
Fatemeh Razzaghi
Directeur de thèse : Dr Hubert Perrot
Présentée et soutenue publiquement le 29 septembre 2016,
Devant un jury composé de :Nicole Jaffrezic DR CNRS Émérite Rapporteur
Francois Tran-Van Prof. Univ. F. Rabelais Rapporteur
Christine Mousty DR CNRS Examinateur Farzaneh Arefi-Khonsari Prof. UPMC Présidente de Jury
Hubert Perrot DR CNRS Directeur de thèse
II
III
Acknowledgements:
First of all, I would like to express my love to my dearest mother for her always unconditional love and support, particularly during my education years in France, it is with no doubt because of her high standard expectations for me if today I stand at this glamourous point in my life. My deepest love and appreciation to my late father, thinking about him at this moment of my life refreshes all the sweet memories of a joyful childhood I spend around him, may his soul rest in peace.
I would like to express my highest regards to my thesis supervisor, Dr Hubert Perrot, for his seriousness, kindness and full support during all the steps of this project from beginning to the end. Today, he is one of the pioneers of EQCM and fast electrogravimetric studies in the country and it was indeed a great honor to have him as my mentor. I will be always grateful for all that I have learned from him.
I am thoroughly grateful for the prosperous collaboration we had with Prof Farzaneh Arefi and Dr Jerome Pulpytel, Plasma field specialists in LISE.
I am really thankful to Dr Christine Mousty (ICCF, Clermont University) for the fruitful collaboration we had regarding the studies on layered double hydroxide (LDHs) materials.
I am indeed grateful to Dr Particia Beaunier for her high quality work and for all that I’ve learned from her in the field of surface classical characterization techniques (SAED, TEM, HR-TEM).
I am thankful to: Dr Francois Huet (LISE) director, Mrs Francoise Pillier (LISE) SEM-FEG Microscopy, Mr Cyril Bazin (LISE) and Mr Benoit Baptiste (IMPMC lab) X-ray Diffractometry analysis, Dr Houssam Fakhouri (LISE) RF reactive Sputtering, Dr Catherine Debiemme-Chouvy (LISE) XPS analysis, Mrs Sandra Casale (LRS) HR-TEM, Mr Axel Desnoyers de Marbaix (LISE) Mecanicien, Mr Daniel Rose (LISE) Electrician, Mrs Florence Billon (LISE) Engineer and Mrs Veronique Martin (LISE) documentalist. During these three years of my PhD project I received the permanent kindness on the behalf of all my friends and colleagues, my kindest regards to Dr Mireille Turmine and Dr Vincent Vivier, my colleague Freddy Escobar, PhD Students: Pierre, Marie S, Thomas, Azadeh, Larbi.
I am also thankful to Miss Marie Brel, our previous M1 student, with whom I have initiated the work on composite materials.
Lastly, I would like to thank all my other friends for their support, special thanks to my lovely sister Maryam.
IV Table of Contents
General Introduction ...1
Résumé du Chapitre I ...3
Chapter I: Bibliography ...6
I. Supercapacitors as Energy Storage Devices ...6
I-1. Basic principle of Electrochemical Supercapacitors ...8
Two types of Electrochemical Supercapacitors ...9
I. 1.1. Electrostatic supercapacitors ...9
I .1.2 Faradaic supercapacitors ...9
II. State of the Art of Different Types of Electrochemical Capacitors (ECs): ... 10
II-1. EC based on Carbon materials ... 11
II-2. Pseudocapacitive oxide materials ... 14
II. 2. 1. WO3... 14
II. 2. 2. TiO2 ... 19
II. 2. 3. MnO2 ... 20
II. 2. 4. RuO2 ... 21
II-3. Conducting Polymers ... 23
II-4. Carbon-MOx Composites ... 26
III. Strategies Toward New Ways of Materials Design for High Performance ECs ... 29
III. 1. Elaboration Methods and Process ... 29
III-2. Materials Nanostructuration for Development of Flexible Device Architectures ... 31
IV. Diagnostics Tools for MOx based Electrodes for Energy Storage ... 34
IV-1. Surface and Structural Analysis: SEM, TEM, XRD, XPS, FT-IR ... 34
(i) SEM images ... 34
(ii) TEM images ... 36
(iii) XRD analysis ... 38
(iv) FTIR methods ... 38
IV-2. Electrochemical Tools ... 39
(i) Cyclic voltammetry ... 39
(ii) EIS technique ... 41
(iii) Quartz Crystal Microbalance ... 43
V. The Scope and Principle Objectives of Thesis Project ... 45
VI. References ... 47
V
Chapter II. Theoretical and Experimental Section ... 60
I. Introduction ... 60
II. Experimental ... 60
II-1. Electrochemical and (Electro)gravimetric Characterization Techniques ... 60
II-1.1. Quartz Crystal Microbalance (QCM) ... 60
II-1.1.1. Principle of QCM ... 61
II-1.1.2. Experimental Set-up ... 63
II-1.2. Cyclic Electrogravimetry (EQCM) ... 63
II-1.2.1. Principle ... 63
II-1.3. Electrochemical Impedance Spectroscopy (EIS) ... 65
II-1.3.1. Principle ... 65
II-1.3.2. Experimental Set-Up ... 67
II-1.4. ac-electrogravimetry; Fast Electrogravimetric Method ... 68
II-1.4.1. Principle ... 68
II-1.4.2. Experimental Method: ∆Vf/∆V ... 69
II-1.4.3. Calibration and the System Corrections ... 71
II-1.4.4. Transfer Function of the Frequency/Voltage Converter: ∆Vf/∆e(ω) ... 72
II-1.4.5. Calibration of the Synthesizer: ∆fs/∆e ... 73
II-2. Preparation of MOx based Thin Films Electrodes ... 74
II-2.1. Elaboration of Compact, Less Porous and Highly Porous TiO2 by RF Reactive Magnetron Sputtering ... 74
II-2.2. Electrochemical Elaboration of Amorphous Compact & Amorphous Mesoporous WO3 ... 76
II-2.3. Electrochemical Elaboration of Compact and Mesoporous RuOx.nH2O ... 79
II-2.4. Elaboration of CNTs/RuOx.nH2O Films ... 81
II-3. Structural and Morphological Investigation Methods ... 81
II-3.1. Scanning Electron Microscopy (SEM) and Energy Dispersive X-rays (EDX) ... 81
II-3.2. Transmission Electron Microscopy (TEM) ... 82
II-3.3. High-Resolution Transmission Electron Microscopy (HR-TEM) ... 83
II-3.4. Selected Area Electron Diffraction (SAED) ... 84
II-3.5. X-ray Diffraction (XRD) ... 86
II-3.6. X-ray Photoelecron Spectroscopy (XPS) ... 87
III. Theory of ac-electrogravimetry ... 89
III-1. A Three Species Model with a Cation, Anion and Free Solvent Contribution ... 89
III-2. Electrochemical reactions and kinetics ... 90
VI
III-4. Charge/ potential transfer function ... 92
III-5. Electrochemical impedance transfer function ... 93
III-6. Mass/potential transfer function ... 94
References ... 95
Résumé du Chapitre III ... 98
Chapter III: TiO2 ... 102
I. Introduction ... 102
II. Structure and Morphology Study of Dense, Less Porous and Highly porous Thin TiO2 Films ... 104
III. Electrochemical Study of Dense, Less Porous and Highly Porous TiO2 Thin Films ... 107
III-1. Cyclic voltammetry and EQCM study in LiClO4 ... 107
III-2. Molecular Mass Estimation ... 108
IV. ac-electrogravimetry studies of TiO2 Thin Films ... 109
IV-1. ac-electrogravimetric studies of Less Porous and Highly Porous TiO2 Thin Films in 0.5M aqueous solution of LiClO4 ... 110
IV-2. ac-electrogravimetric studies of Less Porous and Highly Porous TiO2 Thin Films in 0.5M aqueous solution of NaClO4... 118
IV-3. ac-electrogravimetric studies of Highly Porous TiO2 Thin Films in 0.5M LiClO4, Comparison between Aqueous LiClO4 (aq) and Organic LiClO4 (PC) Solutions ... 125
V. Conclusion ... 133
VI. References ... 135
Résumé du Chapitre IV ... 137
Chapter IV: WO3 ... 141
I. Introduction ... 141
II. Structure and Morphology Study of Dense and Mesoporous Thin Films ... 143
III. Electrochemical studies Study of Dense and Mesoporous WO3 Thin Films ... 146
III-1. The cyclic voltammetry and EQCM study ... 147
III-2. ac-electrogravimetric study of dense and mesoporous WO3 films ... 150
(i) ac-electrogravimetric exploration at -0.3V vs Ag/AgCl ... 150
(ii) ac-electrogravimetric measurements versus potential ... 157
IV. Conclusion ... 161
V. References ... 163
VII
Chapter V: RuOx.xH2O ... 171
I. Introduction ... 171
II. Structural Characterizations ... 172
II-1. XRD Characterization of as-prepared RuOx.xH2O and RuO2 heated at 450 °C ... 172
II-2. SEM-FEG Characterization ... 173
(i) Compact and Mesoporous RuOx.nH2O ... 173
(ii) SWCNTs / RuOx.nH2O ... 175
II-3. Targeting Evolution of Morphology and Crystallography by SAED: The effect of calcination 176 (i) The Effect of Heat Treatment on Compact Ruthenium oxide ... 176
(ii) The Effect of Heat Treatment on Porous Ruthenium oxide ... 176
(iii) The Effect of Structuration on Hydrous Ruthenium Oxide Films ... 177
II-4. HR-TEM Results ... 178
(i) Mesoporous Ruthenium oxide ... 178
(ii) SWCNTs / RuOx.nH2O ... 179
II-5. XPS Characterization, The impact of Heat Treatment ... 179
(i) XPS results of RuCl3.xH2O Precursor ... 180
(ii) RuOx.nH2O and RuO2 ... 182
III. Electrochemical Characterization: The Cyclic Voltammetry and Classical EQCM Study .... 184
III-1. Cyclic electrogravimetry ... 184
III-2. Specific Capacitance Calculation ... 186
III-3. Relative Molecular Mass Estimation ... 188
IV. ac-electrogravimetric results for compact and mesoporous RuOx.nH2O ... 189
IV-1. ac-electrogravimetric results for compact and mesoporous RuOx.nH2O in H2SO4 0.5 M aqueous solutions ... 189
IV-2. ac-electrogravimetric results for compact and mesoporous RuOx.nH2O in Na2SO4 0.5 M aqueous solution ... 198
V. Nanocomposite films of CNTs / RuO2.xH2O Investigated by ac-Electrogravimetric Methods .... 204
V-1. EQCM study of SWCNTs/RuOx.nH2O film in aqueous solution ... 204
V-2. ac-electrogravimetric study of SWCNTs/RuOx.nH2O film in H2SO4 aqueous solution ... 205
VI. Conclusion ... 211
VII. References ... 213
1
General Introduction
During this thesis project, it was attempted to underline the importance of investigating the ion’s exchange phenomena at the metal oxides electrode/electrolyte interface in order to understand and to furtherly improve their promised energetic performances mostly as highly functional electrodes for supercapacitors. Indeed, the key phenomena for all these electrodes functionalities originates in the ion’s exchange at the interface of electrode/electrolyte, it is crucial to investigate the role of electrolyte composition, to identify the status of transferred ions and the solvation effect also to investigate their dynamics of transfer at the interface. Consequently, we have decided to focus on the capabilities of a non-classical methodology so-called, ac-electrogravimetry.
Another subject of crucial matter for our attention was to illustrate the most fundamental reasons of the electrochemical improvements brought by nanostructuration. In fact, the materials structuration with favorable morphologies and unique properties would alter their functionalities and this can effectively be deeply characterized by ac-electrogravimetry. Different morphologies of TiO2, WO3 and RuO2 metal oxides were prepared as furtherly
detailed. As a consequent, during this thesis project, the investigations were performed to see what differences in behavior are brought by procuring porosity within these films. In other words, how mesoporous films with small pore sizes, and large surface area to volume ratios could facilitate the ion intercalation/electroadsorption process involved with our chosen synthesized MOx electrodes. More importantly it was attempted to see how
ac-electrogravimetry as a non-classical coupled methodology would be served for our study to extract fine dynamics details unreachable with classical tools. To pursue this major goal our results were systematically categorized as following:
The first chapter provides a general look toward the highlighted aspects and applications of materials with potential usefulness in energy storage devices. Different examples of each category of materials with emphasizing on MOx based devices are brought to
attention. The recent utile elaboration methods for fabrication of MOx based electrodes
are mentioned. The new ways of materials design in order to upgrade high performance in the electrodes are discussed and finally, the various diagnostics tools for MOx based
electrodes including both structural and electrochemical techniques are introduced. Based on these methodologies the scope of this thesis project which underlines the importance of ac-electrogravimetry as our main chosen methodology is presented.
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The second chapter is dedicated to necessary theoretical and experimental basics commonly used during this thesis project. In the first part, the different components of the model used for ac-electrogravimetric fittings are introduced. It is shown how different ac-electrogravimetric transfer functions for an electroactive electrode system with responses to ions transfer are obtained. Another part is focused on the description of various structural and electrochemical characterization techniques applied as diagnostic tools to study the influence of the different electrochemical and physical preparation methods used for fabricating our MOx electrodes with different
morphologies.
The third chapter underlines the results obtained for different morphologies of amorphous non-porous and amorphous porous TiO2 deposited by reactive magnetron
RF Sputtering. SEM-FEG accompanied with EDX analysis and HR-TEM were used for the structural studies. Both classical EQCM and then, ac-electrogravimetry were used in LiClO4 and NaClO4 aqueous medium to investigate the impact of a porous
morphology and also identify and separate the contribution of the charged and/or uncharged species during the charge compensation process. A comparison is also made to understand the impact of accompanying solvent whether it is organic or aqueous. The fourth chapter includes the results obtained for electrochromic mesoporous WO3
prepared by surfactant-assisted electrodeposition. A comparison in behavior is made between the mesoporous electrode with the compact tungsten trioxide electrode fabricated by electrodeposition but in the absent of surfactant molecules. XPS, SEM-FEG, EDX analysis and HR-TEM were used for the primary structural studies. Then, classical EQCM and ac-electrogravimetry were used in LiClO4 aqueous medium to
investigate the impact of the nanostructuration, and to identify/separate the contribution of the charged and/or uncharged species during the charge compensation process. The last chapter was dedicated to the results obtained for pseudocapacitive hydrous
RuOx.nH2O films prepared in both compact and mesoporous morphologies via the same
electrochemical route used for WO3 electrode preparation. The structure, the
morphology and the composition of these electrodes were analyzed by XRD, XPS, TEM, SAED, HR-TEM and SEM-FEG. Both classical EQCM and ac-electrogravimetric techniques were exploited to study the impact of structuration and the ions transfer behavior of the as-prepared compact and mesoporous hydrous RuOx.nH2O films in aqueous H2SO4 and Na2SO4. The second part of this chapter was
3
pursue the ion’s kinetics evolution involved with a new 3D composite structure in aqueous H2SO4 electrolyte.
Résumé du Chapitre I
Ce chapitre fournit un état de l'art rapide en ce qui concerne les matériaux potentiellement utiles dans les dispositifs de stockage d'énergie, notamment les supercondensateurs. Tout d'abord, le principe des supercondensateurs électrochimiques ainsi que les principales catégories de dispositifs sont brièvement présentées. Deuxièmement, les différentes catégories de matériaux qui présentent de grandes potentialités pour ces dispositifs les oxydes de métaux, les polymères conducteurs, les films carbone / carbone et les composites MOx-carbone, seront
détaillés avec différents exemples pour chacune des catégories. Le sujet de thèse met l'accent principalement sur les systèmes à base de MOx où différents exemples d'oxydes métalliques
sont discutés.
Schéma de principe d'un condensateur électrostatique (A), d'un condensateur à double couche électrique (B), d’un pseudocondensateur (C) et d’un condensateur hybride [9].
Pour améliorer les performances d'électrodes à base d'oxyde métallique, une attention a été portée sur l'effet de la nanostructuration de ces films, selon diverses morphologies et propriétés pour la mise au point de dispositifs de stockage d'énergie électrochimique. De nouvelles
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nanostructures, avec des pores de petite taille et de grands rapports surface sur volume, sont censées faciliter les processus de transfert d'ions à l'interface film structuré/électrolyte. En effet, ce phénomène est essentiel car il intervient dans tous les dispositifs de stockage d'énergie y compris les supercondensateurs. Ces améliorations peuvent être attribuées à (i) un transport facilité avec une très faible longueur de diffusion, (ii) une zone de contact électrode/électrolyte élargie, et (iii) une meilleure gestion du stress mécanique/déformation du matériau lors de l'intercalation/l'électroadsorption d'ions. En conséquence, divers procédés de synthèse ont été développés et sont mentionnés dans une section appropriée.
Dessin idéal d'un matériau d'électrode pour des supercondensateurs de type RuO2.xH2O
nanotubulaires. L'architecture mésoporeuse, la nature hydratée des surfaces, et la conductivité métallique fournissent aux protons et aux électrons des "autoroutes" qui doivent favoriser les
différents processus électrochimiques et ainsi permettre d'obtenir des condensateurs très efficaces [68].
Dans une seconde partie de ce chapitre I, les différentes techniques d'imagerie et les outils de
diagnostic électrochimique pour des électrodes à base de MOx sont présentés.
Image HR-TEM d'un échantillon RuO2·xH2O synthétisé en utilisant un agent tensio-actif pour
5
Une méthodologie attractive pour étudier l’intercalation / l'électroadsorption des ions au sein de ces matériaux est basée sur des dispositifs de type microbalance à quartz (QCM). Il y a eu diverses études utilisant une microbalance à quartz comme sonde gravimétrique in-situ afin d'étudier ces phénomènes au sein d'oxydes métalliques ou d'électrodes à base de carbone.
Caractérisation EQCM d'un film de Li-MnO2 de type Birnessite dans des électrolytes aqueux de
LiClO4 et NaClO4 [107].
Malgré les informations utiles données par EQCM lorsqu'il s'agit de décrire les mécanismes d’intercalation ou d’électroadsorption ioniques, impliqués lors du stockage de charges dans des matériaux capacitifs, quelques limitations apparaissent. Les microbalances à quartz classiques donnent une réponse globale correspondant en fait à plusieurs processus possibles et plus ou moins simultanés. Des ions nus, des ions avec des coquilles de solvatation et des molécules de solvant libre peuvent ainsi être associés, directement ou indirectement, aux différents processus électrochimiques. En outre, les ions peuvent perdre une partie de leur solvatation pour accéder aux sites situés dans les plus petits pores. Ces différentes voies possibles associées à des aspects cinétiques n'ont jamais été suivies à l'aide de dispositifs classiques d'EQCM.
L’objectif majeur de ce projet de thèse est de mettre en valeur un outil de caractérisation alternatif afin de surmonter les limitations des EQCM classiques pour étudier les mécanismes d'électroadsorption/d'intercalation dans les électrodes associées aux systèmes pseudo-capacitif. Cette méthode, dite d'électrogravimétrie à courant alternatif ou ac-électrogravimétrie est constituée d'un couplage entre la spectroscopie d'impédance électrochimique (EIS) et une
6
microbalance à quartz rapide (QCM). Cette approche a été développée dans un nombre limité de laboratoires dans le monde.
Chapter I: Bibliography
This Chapter provides an introduction to the highlighted aspects and applications of materials with potential usefulness in energy storage devices, more importantly supercapacitors and/or micro-capacitors. First, the functionality of electrochemical supercapacitors and their main categories are briefly introduced. Secondly, different categories of materials with great potentialities in these devices are mentioned including metal oxide, conducting polymers, carbon/carbon and MOx-Carbon composites based supercapacitors. Different examples of each
category are brought to attention. Since the thesis subject emphasizes mainly on MOx based
devices, a few examples of different metal oxides are also discussed. Then, the recent utile elaboration methods for fabrication of MOx based electrodes are mentioned. Another
alternatively interesting followed aspect is the new ways of materials design in order to upgrade high performance in the electrodes. Finally, the various diagnostics tools for MOx based
electrodes including both structural and electrochemical techniques are introduced and based on these methodologies the scope of this thesis project is presented.
I.
Supercapacitors as Energy Storage Devices:
With the rapid development of the global economy, the depletion of fossil fuels, and increasing environmental pollution, there is an urgent need for efficient, clean, and sustainable sources of energy, as well as new technologies associated with energy conversion and storage. In many application areas, some of the most effective and practical technologies for energy conversion and storage are fuel cells and batteries/electrochemical supercapacitors. In recent years, electrochemical supercapacitors have attracted significant attention, mainly due to their high power density, long lifecycle, and bridging function for the power/energy gap between traditional dielectric capacitors (which have high power output) and batteries (which have high energy storage). The earliest electrochemical supercapacitors patent was filed in 1957. However, not until the 1990s did electrochemical supercapacitors technology begin to draw some attention, in the field of hybrid electric vehicles. It was found that the main function of electrochemical supercapacitors could be to boost batteries or fuel cells in a hybrid electric vehicle to provide the necessary power for acceleration, with an additional function being to recuperate brake energy [1–10]. Further developments have led to the recognition that
7
electrochemical supercapacitors can play an important role in complementing batteries or fuel cells in their energy functions by providing back-up power supplies to protect against power disruptions. As a result, the US Department of Energy has designated electrochemical supercapacitors to be as important as batteries for future energy storage systems. Many other governments and enterprises have also invested time and money into exploring, researching, and developing electrochemical supercapacitors technologies [1–10].
Recent years have yielded major progress in the theoretical and development of electrochemical supercapacitors, as evinced by a large number of research articles and technical reports [4–10]. At the same time, the disadvantages of electrochemical supercapacitors including low energy density and high production cost have been identified as major challenges for the furtherance of electrochemical supercapacitors technologies. To overcome the obstacle of low energy density, one of the most intensive approaches is the development of new materials for electrodes. Most popular today are carbon particle materials, which have high surface areas for charge storage. But in spite of these large specific surface areas, the charges physically stored on the carbon particles in porous electrode layers are unfortunately limited. Electrochemical Supercapacitors (ES) of this kind, called Electrical Double-Layer Supercapacitors (EDLS), have a limited specific capacitance (measured in Faraday per gram of the electrode material) and a low electrochemical supercapacitors energy density.
Advanced approaches to increase the ES energy density are to hybridize the electrode materials by adding electrochemically active materials to a carbon-particle-based ES electrode layer or to completely replace the carbon materials with another electrochemically active material. ES with electrochemically active materials as electrodes are called pseudo-supercapacitors [4]. It has been demonstrated that faradaic or hybrid double-layer supercapacitors can yield much higher specific capacitance and electrochemical supercapacitors energy density than EDLS. Regarding advanced electrochemical supercapacitors materials, metal oxides such as ruthenium oxides and manganese oxides are considered the most promising materials for the next generation of electrochemical supercapacitors. Therefore, in this project we pay particular attention to metal oxides and their applications in electrochemical supercapacitors electrodes. First, however, we provide some introductory background on electrochemical supercapacitors, which we hope will facilitate our review and analysis of the literature. Finally, we will discuss the direction that future research in electrochemical supercapacitors might be expected to take.
8
I-1. Basic principle of Electrochemical Supercapacitors
An electrochemical supercapacitor is a charge-storage device similar to batteries in design and manufacturing. As shown in Fig. I-1, an ES consists of two electrodes, an electrolyte, and a separator that electrically isolates the two electrodes. The most important component in an electrochemical supercapacitor is the electrode material. In general, the ES’s electrodes are fabricated from nanoscale materials that have high surface area and high porosity. It can be seen from Fig. I-1 that charges can be stored and separated at the interface between the conductive solid particles (such as carbon particles or metal oxide particles) and the electrolyte. This interface can be treated as a capacitor with an electrical double-layer capacitance, which can be expressed as the following equation:
𝐶 =𝐴𝜀
𝑑
where A is the area of the electrode surface, which for a supercapacitor should be the active surface of the electrode porous layer; 𝛆 is the medium (electrolyte) dielectric constant, which will be equal to 1 for a vacuum and larger than 1 for all other materials, including gases; and d is the effective thickness of the electrical double layer.
Fig. I-1. Principles of a single-cell double-layer capacitor and illustration of the potential drop at the
electrode/electrolyte interface [11].
As described in the Introduction, two types of ES exist. One is the EDLS, in which the electrode material, such as carbon particles, is not electrochemically active. In other words, there is no electrochemical reaction on the electrode material during the ES charging and discharging
9
processes, and pure physical charge accumulation occurs at the electrode/electrolyte interface. The other type is the faradaic supercapacitors, in which the electrode material is electrochemically active, e.g. metal oxides, which can directly store charges during the charging and discharging processes.
Two types of Electrochemical Supercapacitors
I. 1.1. Electrostatic supercapacitors
The capacitance of the electrode/interface in an electrostatic or EDLS is associated with an electrode-potential-dependent accumulation of electrostatic charge at the solid/electrolyte interface. As shown in Fig. I-1, this electrical double-layer capacitance comes from electrode material where an excess or a deficit of electric charges can be accumulated on the electrode side as electrolyte ions must counterbalance them on the electrolyte side in order to meet electroneutrality.
During the process of charge, the electrons travel from the negative electrode to the positive electrode through an external load. Within the electrolyte, cations move towards the negative electrode while anions move towards the positive electrode. During discharge, the reverse processes take place. In this type of supercapacitors, no charge transfer across the electrode/electrolyte interface is observed and no net ion exchanges occur between the electrode and the electrolyte.
I .1.2 Faradaic supercapacitors
Faradaic supercapacitors or pseudocapacitors are different from electrostatic or EDLS. When a potential is applied to a faradaic supercapacitor, fast and reversible faradaic reactions (redox reactions) take place on the electrode materials and involve the passage of charge across the interface, similar to the charging and discharging processes that occur in batteries, resulting in faradaic current passing through the supercapacitor cell. Materials undergoing such redox reactions include conducting polymers and several metal oxides, including RuO2, MnO2, or
Co3O4. Three types of faradaic processes can occur at faradaic supercapacitors electrodes: direct
electrochemical reactions it is a little beat confused as in EDLC it is electro adsorption (for example, adsorption of hydrogen on the surface of platinum or gold), redox reactions of transition metal oxides (e.g. RuO2), and reversible electrochemical doping–dedoping in
conducting polymer based electrodes. It has been demonstrated that these faradaic electrochemical processes not only extend the working voltage but also increase the specific
10
capacitance of the supercapacitors. Since the electrochemical processes occur both on the surface and in the bulk near the surface of the solid electrode, a faradaic supercapacitor exhibits far larger capacitance values and energy density than an EDLS. As reported by Conway et al. [11] the capacitance of a faradaic supercapacitors can be 10–100 times higher than the electrostatic capacitance of an EDLS. However, a faradaic supercapacitor usually suffers from relatively lower power density than an EDLS because faradaic processes are normally slower than nonfaradaic processes. Moreover, because redox reactions occur, faradaic supercapacitors often lack stability during cycling, similarly to batteries. It is worth mentioning that hybrid faradaic supercapacitors with an asymmetrical electrode configuration (e.g. one electrode consists of electrostatic carbon material while the other consists of faradaic capacitance material) have been extensively studied recently to capitalize on both electrode materials advantages in improving overall cell voltage, energy, and power densities. In this kind of hybrid supercapacitor, both electrical double-layer capacitance and faradaic capacitance mechanisms occur simultaneously, but one of them plays a greater role. The different configurations are presented in figure I-2.
Fig. I-2. Schematic diagram of (A) an electrostatic capacitor, (B) an electric double-layer capacitor,
(C) a pseudocapacitor, and (D) a hybrid-capacitor [9].
II. State of the Art of Different Types of Electrochemical Capacitors
(ECs):
The most promising materials for electrochemical supercapacitors should utilize both the fast and reversible faradaic pseudocapacitance coming from the redox transitions of the interfacial electroactive species and the indefinitely reversible capacitance of electric double-layer formed at the electrolyte–electrode interface. Based on this point of view, the electrochemical
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supercapacitors can be considered as a hybrid device exhibiting transitional behavior between batteries and capacitors. Potential candidates of electrodes should be consisted of electroactive materials with very high surface areas, which prosecute the fast reversible redox transitions in the potential window of charge and discharge.
ECs are intermediate systems between dielectric capacitors and batteries. While batteries able to store higher energy density than supercapacitors, they deliver less power; as compared to dielectric capacitors, supercapacitors can store higher energy density with less delivered power. These particular properties make them suitable for numerous applications such as power electronics, spatial or military field; they can also be used in hybrid electric vehicle in order to help the stop and go function, to provide peak power for improved acceleration, for energy recovery.
Three main classes of supercapacitors are described in the literature: metal oxide [12–22], electronically conducting polymer [23–30] and Carbon/Carbon supercapacitors [31,32– 37]. Recently, hybrid supercapacitors have been developed where an activated carbon electrode is associated with a faradic electrode [38–53].
II-1. EC based on Carbon materials:
Carbon/Carbon supercapacitors have been largely investigated because of their low-cost, high cycling-life and high capacitance. Small (few farads) up to large-size (5000 F) devices are commercially available (Maxwell, Epcos, Panasonic, etc.). Highly-porous carbons are used as electrode material due to their high surface area, good electronic conductivity and high electrochemical stability; the most frequently used is activated carbon (1500–2000 m2 g−1).
Charge storage is performed through the reversible electro adsorption of the ions at the active material/electrolyte interface; no faradaic reactions occur during the charge–discharge of the supercapacitor [54].
Carbon materials, such as activated carbons (ACs) and carbon nanotubes (CNTs) (Fig. I-3), usually exhibit good stability but limited capacitance values. It is clear that EDLCs processes are surface phenomena, and hence the performance greatly depends on the electrolyte-accessible surface area. The micropores in carbon materials are inelectrolyte-accessible by the electrolyte, resulting in the inability of the double layer to be formed inside the pores. This result leads to a decrease in the capacitance value (10–20% of the ‘theoretical’ capacitance) of ACs. Good electrical conductivity, high chemical and mechanical stability, and an optimized nanostructure are also other important factors that are responsible for achieving high capacitance values. The
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potential windows in nonaqueous electrolytes are generally larger than those obtained in aqueous media, although the overpotential of hydrogen evolution on carbons is high, especially in neutral electrolytes.
Fig. I-3. The most common carbon materials classified based on their bonding (hybridization of orbitals
of carbon atoms) and dimensionality (i.e., the number of dimensions not confined to the nanoscale). Whereas graphite, carbon fibers, glassy carbon, activated carbons, carbon black, and diamond are already widely used in industry, fullerenes and fulerides, carbon onions (multishell fullerenes), nanotubes, whiskers, nanofibers, cones, nanohorns, nanorings, nanodiamonds, and other nanoscale carbons are being explored for future technologies. Note: 0D, zero-dimensional; 1D, one-dimensional; 2D, two-dimensional; 3D, three-dimensional [10].
The most recent advances in supercapacitor materials include the development of nanoporous carbons with the pore size tuned to fit the size of ions of the electrolyte with Ångström accuracy. In a recent review by P. Simon and co-workers [55] it was attempted to achieve an improved understanding of charge storage and ion desolvation in sub-nanometer pores. It has also stressed the requirement of matching the active materials with specific electrolytes and the need to use a cathode and anode with different pore sizes. The very large number of possible active materials and electrolytes requires better theoretical guidance for the design of more powerful and long-lasting EDLCs (Fig. I-4).
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Fig. I-4. Capacitance change versus current density for laboratory cells assembled with CDCs with
various pore sizes [55].
Many attempts have been made to increase the specific capacitance of carbon nanotubes (CNTs). Electrochemical enhancement by adding redox active functional groups on CNTs increases the specific capacitance, while excessive oxidation decreases conductivity and leads to poor cycle life. X. Xiao et al. [37] reported on the electrochemical enhancement methods followed by annealing at different temperatures in air to add and adjust the redox active functional groups. Functionalized freestanding CNT films were used as positive electrodes, assembled with freestanding CNT/MoO3_x negative electrodes to fabricate carbon
nanotube-based solid-state asymmetric supercapacitors (ASCs).
Fig. I-5. (a) Schematic of the fabrication procedure for functionalized free standing CNT films. (b) Cross
section SEM image of the functionalized freestanding CNT film. (c) Enlarged cross-section SEM image of the functionalized free standing CNT films. The inset is a digital image of a functionalized freestanding CNT film [56].
J Tang et al. [56] synthesized of nitrogen-doped hierarchical porous carbons and by this way a three-dimensional interconnected framework (NHPC-3D) was developed (Fig. I-5). This device
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showed a high capacitance retention of 75,7% at a higher current density of 20 Ag-1 in aqueous
electrolyte. This value is due to a sufficient surface area for charge accommodation, reversible pseudocapacitance, and a minimized ion-transport resistance, as a result of the advantageous interconnected hierarchical porous texture.
II-2. Pseudocapacitive oxide materials
In energy sources, there is a need, particularly with transportation and grid storage applications, of a large amounts of energy delivered quickly, within seconds or minutes. Although carbon based electrochemical capacitors possess the required power density, their relatively low energy density limits their usefulness for these applications. Instead, transition metal oxides that exhibit pseudocapacitance are very attractive. Pseudocapacitance occurs when reversible redox reactions occur at or near the surface of an electrode material and are fast enough so that the device's electrochemical features are those of a carbon-based capacitor, but with significantly higher capacitances. It is important to recognize that pseudocapacitance in materials is a relatively new property, with the first materials identified in the 1970's. To date, transition metal oxides exhibit the widest range of materials with pseudocapacitive behavior. By selecting the proper transition metal oxide, utilizing the most effective electrode architecture, and analyzing the electrochemical behavior for pseudocapacitive behavior, such materials are expected to become the basis for electrochemical energy storage devices which can offer high energy density. The following part involves several examples of various pseudocapacitive metal oxides as potential materials for energy storage.
II. 2. 1. WO
3Tungsten oxides are great electrochemically active materials for energy storage. They have intriguingly fast and reversible surface redox reactions, which is associated with the similar W−O bond lengths in tungsten oxides with different oxidation states. Furthermore, the intrinsic high densities of tungsten oxides show potential applications in the fabrication of compact devices with excellent power performance. Theoretically, tungsten oxides are formed by corner and edge sharing of the WO6 octahedra. By ordered stacking of WO6 octahedra, a considerable
number of interstitial sites can be formed. Such interstitial sites are effective accommodations for guest ions, rendering the absorption and desorption of ions at the surface, as well as insertion and deinsertion into the inner parts [57].
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Table I-1. Comparison of the Performances of Supercapacitors Based on Tungsten Oxides and Other
Popular Active Materials [57].
In a recent paper by M. Zhu et al. [57], it is reported that proton insertion, a faradaic mechanism, may effectively enhance the global capacitance of metal oxides with low surface area but specific structures. According to this report performance could be improved when an assembly structure of hexagonal phase WO3 (h-WO3) nanopillars is synthesized which favors enhanced
proton insertion mechanism. As it is demonstrated in Fig. I-6 d high capacitance of up to 422 F g−1 under the current density of 0,5 A g−1 could be achieved by this electrode (Table I-1).
Fig. I-6. CV curves (a), profiles of capacitance versus potential (b), and charge−discharge profiles (c)
measured at different rates and (d) cycling stability of the as-synthesized h-WO3 nanopillars [57].
Recently, J. Xu et al. [58] reported on the fabrication of WO3 mesoscopic microspheres
composed of nanofibers and prepared via a hydrothermal process (Fig. I-7). Then, an asymmetric supercapacitor was constructed using these as-prepared WO3 mesoscopic
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electrochemical performances are attributed to the mesoscopic structure of the WO3 which
consist of a self-assembled interconnected WO3 nanofibers framework. Thus, films have open
porous characteristics and abundant active sites accessible to charge storage leading to high specific capacitance of 797 F g-1 at a current density of 0,5 A g-1. The specific capacitance
reported by author is much higher than that of tungsten oxide materials.
Fig. I-7. (a) Low and (b) High magnification SEM images; (c) TEM image of a single WO3 microsphere; (d) HRTEM image of WO3 nanofibers. The inset is selected area electron diffraction (SAED) of the selected area in (d) [58].
Nano-WO3.H2O/MnO2 was synthesized on Ti/RuO2+TiO2 substrate via anodic
electrodeposition by C. Yuan et al. [59]. The advantageous aspects of this new nanostructure is firstly related to good electron conduction of the nano-WO3-H2O due to MnO2. In their study,
tungsten oxide had a conductivity of 1,76 S cm-1 which is several orders of magnitude higher
than that of MnO2 (10-5 – 10-6 S cm-1). Secondly, thin layer of MnO2 on each nanosheet enables
fast faradic reaction and provides a short ion diffusion path and therefore improved capacitive performance, finally, the porous structure can create channels for effective transport of electrolyte and consequently increase electrochemical active sites.
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Fig. I-8. (a) Cyclic voltammograms of Ti/RuO2+TiO2 substrate, nano-WO3-H2O, MnO2 and NWMO within a potential window of -0.1 – 0.9 V at 5mVs-1 scan rate (b) cyclic voltammograms of NWMO at different scan rates [59].
Based on their electrochemical characterizations (Fig. I-8), this new electrode exhibits excellent capacitive performance including high specific capacitance of 363 F g-1 and long-term stability,
which is resulted from this special nanostructure [59].
In another work reported by Z Chen et al. [21] a mixed protonic-electronic conductor was synthesized by building the proton conducting water chains within a matrix of electron-conducting hydrous hexagonal tungsten oxide (h-WO3) (Fig I-9).
Fig. I-9. (a) SEM image of as-synthesized h-WO3 nH2O particles. (b) High-resolution TEM image and fast Fourier transform (FFT) images (inset) of a single h-WO3 nH2O nanorods [21].
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The mixed conductivity affords the h-WO3·nH2O with unprecedented energy storage
performance. Cyclic voltammograms (CVs) in Figure I-10 show highly reversible peaks of reduction (1/1′) and oxidation (2/2′) reactions in acidic electrolyte. These reversible peaks are responsible for progression of reduction/oxidation of tungsten accompanied by proton insertion into the crystal structure from surface sites and through channels, respectively. The “mirror-like” CV curves reveal a capacitive behavior, which is commonly observed for hydrous RuO2
the best-performing pseudocapacitive materials known so far. By comparison, in Li2SO4 and
Na2SO4 electrolytes the voltametric current are smaller, and the electrodes displayed featureless
CV response over the whole potential range, suggesting H+ insertion/extraction is the most
favorable process for h-WO3·nH2O.
Fig. I-10. Electrochemical performance of h-WO3·nH2O electrodes. (a) CV curves of h-WO3·nH2O electrodes at voltage sweep rate of 10 mV s−1 in 0.2 M H
2SO4, Li2SO4 and Na2SO4 electrolytes in three-electrode cells with Ag/AgCl and Pt foil as the reference and counter three-electrode, respectively. (b) Comparison of electrochemical capacitive performance of our h-WO3·nH2O with other WO3 structures (h-WO3 with CNT, h-WO3 bundle structure, hexagonal-cubic mixed phase WO3, monoclinic WO3 thin film) [21].
The high rate performance is mainly due to the coexistence of high ionic and electronic conductivity in the hierarchical h-WO3·nH2O crystal structure. Besides the high mass-, area-,
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unique hierarchical nanostructure with proton-conducting channels also endows the h-WO3·nH2O with outstanding cycling stability.
II. 2. 2. TiO
2Using TiO2 anatase nanosheets interfaced with lithium ion-containing electrolytes as an
example, J Kang et al. [60] revealed a microscopic mechanism for lithium intercalation in this system. They have demonstrated that a TiO2 nanosheet is a hybrid between supercapacitor and
battery, possessing characteristics of both depending on the electrode potential. At positive electrode potential above 2.2 V versus Li/Li+, the system behaves as a capacitor with the
formation of electric double layers at the surface. As the electrode potential decreases below the threshold, lithium intercalation into the interior takes place. Their findings provide a coherent picture of how a transition from pure capacitors to batteries or pseudocapacitors occurs in these nanostructured materials.
In another work by T. Brezesinski et al. [61], the considerable enhancement of the electrochemical properties of self-organized TiO2 results are shown when the films are both
made from nanocrystals and mesoporous structures. Such materials show high levels of capacitive charge storage and high insertion capacities. When nanocrystal-based films are formed without mesoscale porosity, a high fraction of the total stored charge is again capacitive, but the total capacity is low, likely because most of the film is not accessible to the electrolyte/solvent. By contrast, when mesoscale porosity is created in a material with “dense” walls (rather than porous walls derived from the aggregation of nanocrystals), insertion capacities comparable to templated nanocrystal materials can be achieved, but the capacitance is much lower. These results underscore the importance of pseudocapacitive behavior which is developed in high surface area mesoporous oxide films. Moreover, the data suggest that both a mesoporous morphology and the use of nanocrystals as the basic building blocks are very promising for the rational development of metal oxide pseudo/supercapacitors. Through this combination, it may become possible to attain greater power densities while maintaining energy density in the next-generation of electrochemical capacitors that utilize this bimodal nanoporous architecture.
X. Sun et al. [62] fabricated pseudo supercapacitors by depositing amorphous TiO2 thin films
onto the surface of both graphene and CNT samples by atomic layer deposition (ALD). An ultrathin Al2O3 adhesion layer was employed to obtain conformal and amorphous TiO2 films.
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using a variety of techniques including cyclic voltammetry, galvanostatic charge/discharge curves, and electrochemical impedance spectroscopy. The relatively low electrical conductivity and ionic diffusivity of TiO2 did not limit these measurements because the TiO2 films were
ultrathin. The pseudocapacitance of the TiO2 films greatly exceeded the electric double layer
capacitance of the uncoated graphene and CNT samples. The measurements determined that the specific capacitances of the TiO2-coated graphene and CNT samples were 97,5 and 135 F
g-1, respectively, after 50 TiO2 cycles at 1 A g-1. An asymmetric cell was also developed based
on TiO2 ALD coated CNT samples as the positive electrode and uncoated CNT samples as the
negative electrode. This energy storage device could be reversibly operated over a wide voltage range of 0−1,5 V in aqueous electrolyte. A high energy density of 4,47 W·h kg-1 was achieved
on the basis of the total weight of both electrodes. This energy density was ∼4 times higher than the symmetric CNT cell. The TiO2 ALD-coated G and CNT electrodes and the asymmetric
cell based on the TiO2 ALD coated electrode also exhibited excellent stability over >1000
cycles. The results of this study demonstrate that metal oxide on high surface area conducting substrates can be used to fabricate high energy storage supercapacitors.
II. 2. 3. MnO
2The pseudocapacitive behavior of MnO2 was first investigated in 1999 by Lee and Goodenough
[63–66] as they studied the properties of amorphous MnO2.nH2O in a KCl aqueous electrolyte.
The presence of a rectangular voltammogram and the storage of approximately 200 F g-1 (240
C g-1, 4 min) indicated that rapid faradaic reactions were responsible for charge storage in this
material.
Q Qu et al. [67] investigated the electrochemical behavior of MnO2 nanorods prepared from a
precipitation reaction in 0,5 mol L-1 Li2SO4, Na2SO4, and K2SO4 aqueous electrolytes (Fig.
I-11). The MnO2 nanorods show the superior rate behavior in the K2SO4 electrolyte due to the
smallest hydrated radius of K+, highest ionic conductivity, and smallest equivalent series
resistance. Moreover, the initial cycles could be an effective way to activate the MnO2 electrode
and make the electrolyte solution soak into the material surface completely. The assembled asymmetric AC/K2SO4/MnO2 nanorods supercapacitor shows an excellent cycling behavior
between 0 and 1,8 V and also exhibits a large energy density of 17 Wh kg-1 at a power density
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Fig. I-11. Capacitance variations of MnO2 nanorods at different scan rates in 0,5 mole L-1 aqueous Li2SO4, Na2SO4, and K2SO4 electrolytes [67].
Layered δ-MnO2 thin films, featuring 3-D porous morphology and high hydrophilicity, were
fabricated directly on inexpensive stainless steel (SS) foil substrates through a chemical bath deposition method (CBD) at low temperature by Y. Hu et al. [66]. The δ-MnO2/SS electrode
exhibits a high Csp of 447 F g-1 at 2 mV s-1 and maintains 264,4 F g-1 even at the high scan rate
of 500 mV s-1. A good capacitance retention ratio of 87% is obtained after 1000 CV cycles at
10 mV s-1 in 0.5 M Na2SO4. The δ-MnO2/SS electrodes also exhibit excellent mechanical
flexibility and electrochemical stability after 200 bending cycles. The capacitance of the thin film electrodes becomes better with the operating temperature rising up from 10 to 45°C and the capacitance retention ratio can still remain up to 97,9% at 10°C. These results demonstrate that the δ-MnO2 thin films have potential applications as electrode materials for flexible ECs.
II. 2. 4. RuO
2In 1971, a new type of electrochemical capacitance was discovered using RuO2, termed
pseudocapacitance because it involved faradaic charge-transfer reactions. The storage of protons from the electrolyte resulted in a faradaic charge-transfer reaction on the RuO2 thin film
electrode. Despite the faradaic nature of the charge storage process, the cyclic voltammogram (CV) was that of capacitor i.e. rectangular in shape (Fig. I-12) as electroadorsption phenomena also remain. While this first report resulted in low gravimetric capacitance values (only 4–7% of the Ru4+ atoms participated in the redox reaction), it demonstrated the unique electrochemical
features of pseudocapacitive processes. This study also demonstrated the need for a porous and hydrous oxide as the bulk, single-crystal material did not exhibit a rectangular CV. Subsequent studies improved the capacitance to over 700 F g-1 by identifying the importance of structural
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0,5H2O exhibits four unique features that enable rapid faradaic reactions with high capacitance:
(1) the redox behavior of the Ru4+ cation that allows for faradaic energy storage; (2) the metallic
conductivity of RuO2 that allows for rapid electron transport; (3) the presence of structural water
that enables rapid proton transport within the so-called “inner surface”; and (4) a large “outer” surface area that decreases diffusion distances. Unfortunately, the high cost of ruthenium (currently at ~2000 USD per kg) makes devices based on RuO2 impractical for widespread
application, except in small-size devices such as microsupercapacitors. Nevertheless, the behavior of hydrous RuO2 first demonstrated that in certain systems, reversible faradaic
reactions can result in similar electrochemical features as those of a capacitor. The study of RuO2 also led to the understanding of what constitutes an ideal pseudocapacitive material in
aqueous electrolytes.
Fig. I-12. The general electrochemical features of pseudocapacitive materials. (a) In a cyclic
voltammetry experiment, the shape is rectangular and if peaks are present, they are broad and exhibit a small peak-to-peak voltage separation. (b) In a galvanostatic experiment, the shape is sloping so that a capacitance value, dQ/dE, may be assigned at each point, and the voltage hysteresis is small. Here, Q is the capacity and E is the potential window. (c) In an AC impedance experiment, the Nyquist representation will contain a vertical line with a phase angle of 90° or less. A semi-circle at high frequencies, associated with charge-transfer resistance, may also be present [27].
Both amorphous and microcrystalline ruthenium oxides have been employed as electrode materials for the EC supercapacitors although ruthenium is a noble metal. The kinetics of electron transfer and/or the diffusion of protons within the electroactive materials determined the reversibility of redox transitions. An integration relating the electrochemical characteristics and textural properties to their preparation methods are very important in searching and optimizing the suitable materials for the application of EC supercapacitors [68–70]. Even though RuO2 has a great advantage in terms of attractive high specific capacitance, it is
somewhat expensive for commercial electrochemical capacitors. However, if one would be able to develop a thin film process for preparing RuO2 electrode, this would reduce significantly
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Hu et al. [70] produced nanotubular hydrous RuO2 electrodes using the anodized aluminium
oxide (AAO) membrane template and showed a specific capacitance of 1300 Fg−1. Zheng et al.
[71] synthesized hydrous RuO2 films through a sol–gel process and showed specific capacitance
depending on hydrate species contents and surface area. A maximum specific capacitance of 720 Fg−1 was obtained for RuO2·0,5H2O with a surface area of 68,6 m2 g−1. Suh et al. [72]
produced high surface area RuO2 aerogels of 350 m2 g−1 with the high hydrate contents of
RuO20.5H2O by carbon dioxide supercritical drying method and showed a specific capacitance
of 595 Fg−1. Subramanian et al. [73] prepared the mesoporous anhydrous crystalline RuO2 by
using nonionic surfactant template and achieved a specific capacitance of 58 Fg−1. In RuO2 as
a pseudocapacitor, in which both electrons and cations have to be transported, facile transport paths for both types of carriers should exist. Anhydrous RuO2 has the rutile structure and is
composed of RuO6 octahedra in the three-dimensional structure, the ordering of RuO6 octahedra
is able to promote electron transport, but not cation transport. However, hydrous RuO2 may
comprise the chains of disordered RuO6 octahedra and exhibit the three-dimensional disorder,
which is able to promote cation transport, but less electron transport. Therefore, pseudocapacitance should be optimized and increased when the hydrated contents and ordered RuO6 octahedra in amorphous RuO2 are balanced. The control of the hydrated contents of
(RuO2·xH2O) in the porous nanoarchitectures of RuO2 is an important parameter to obtain a
high specific capacitance.
In a recent study in this regard, H-S Nam et al. [74] proposed a new process for preparing nanoporous and hydrous RuO2 using sodium dodecyl sulfate (SDS) at a low temperature below
100◦ C. Their resulted nanoporous and hydrous RuO2 displayed a good specific capacitance
behavior. The proposed nanoporous RuO2·3.38H2O and RuO2·2.56H2O showed maximum
specific capacitances, i.e., 870 Fg−1 and 833 Fg−1, respectively, at a scan rate of 10 mV s−1, and
exhibited pure electrochemical capacitive behavior. From their results it is suggested that the nanoporous ruthenium oxide with more hydrated is more suitable for high performance capacitors than other materials.
II-3. Conducting Polymers:
Generally, ECs performance is considered in specific capacitance which either can be estimated from the cyclic-voltammogram or from the discharge measurement. Each class of materials has its unique advantages and disadvantages. Some of the transition-metal oxides suffer from poor electrical conductivity and high cost. On the other hand, conducting polymers are highly
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conducting, can follow easy synthesis process, are associated to fast rate of redox transfer and present low cost, etc. But their poor cycle life and low mechanical stability are major barriers for practical applications. Therefore, many studies have recently been addressed to increase their electrochemical performances [24–27, 31].
Various research groups are trying to investigate a hybrid nanostructure containing RuO2 and
conducting polymers such as polyaniline, polypyrrole etc., for ECs application. Employing template-free anodic deposition of cone-shaped nanostructure of polypyrrole with ultrathin layer of RuO2, a specific capacitance of 302 Fg-1 has been reported. Polyaniline/nafion/hydrous
RuO2 ternary composite electrode has demonstrated a 475 Fg-1 specific capacitance at 100 mV
s-1 [24–27,31].
In recent years, polypyrrole (PPy), an important conducting polymer, has been successfully employed as redox electrode material. The specific capacitance of PPy has been measured to be 180–250 Fg-1. In spite of its high charge storage capacitance, PPy and other conducting
polymers lack in long-term stability [24–31].
In a report from F. Tran-Van’s group, the capacity of chemically synthesized PPy, as well as its stability during cycling was optimized. In their work [28,29] polymerization of pyrrole is performed in a colloidal solution of Fe2O3 and in the presence of PTS anions. In this conditions,
improvements are reached in terms of the charge storage capacity of the material. This is attributed to a morphological modification of the composite. Nanoparticles role as a support to the polymerization process of pyrrole and eventually this leads to a more porous structure with a higher specific surface area. By this way, it increases the accessibility of the conducting polymer sites during the electrochemical process. As a consequence, the kinetics of charge transfer is altered amplified in this nanocomposite structure. The insertion–extraction of anions during the charge–discharge process was reported to be easier and it may allow higher rates of charge–discharge and higher power density to be obtained.
S. F. Shaikh et al. [25] reported on the superior electrochemical supercapacitive performance of hybrid structure over organic and inorganic structures (Fig. I-13). According to their work electrodeposited H-RuO2 (hybrid nanostructure electrode) plays an important role for
transportation of ions through PANI films in acidic electrolyte. Presence of the H-RuO2
electrode enhanced length of PANI nanowires. Hybrid structure is not only reducing diffusion resistance of electrolyte in the electrode material but also increasing specific capacitance. The formation of hybrid H-RuO2 and PANI structure is confirmed from the Raman study. The
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obtained specific capacitance of 322 F g-1 at 50 mV s-1 scan rate for this hybrid electrode is
promising.
Fig. I-13. Scanning Electron Microscopy images of; a) H-RuO2, b) PANI, c) RP30 (Electrodeposition of PANI was performed at a constant potential of 0.7 V for 30 min at room temperature) and d) X-ray diffraction patterns of electrodes [25].
B-X Zou et al. [75] havereported on the pseudocapacitive properties of WO3/PANI composite
films electrodeposited by cyclic voltammetry in solutions of aniline and precursor of the oxide prepared from tungstic acid and H2O2. These composite films exhibited good pseudocapacitive
performance in a wide potential range of -0,5 to 0,7 V vs. SCE due to the electroactivities of WO3 in negative potential range and its fine distribution in PANI matrix. Fig. I-14 presents the
specific capacitance of WO3, PANI and WO3/PANI composite (WP) with different mass of
films. It is assumed that with the mass of films increasing, the specific capacitance of WO3
decreases. This is attributed to the diffusion limitation in the WO3 dense film. Due to the
distribution of WO3 in the PANI network, WO3/PANI composite displays only a minor decrease
in the specific capacitance, with the mass of film increasing from 2,4 to 8,1 mg. This implies that the pseudocapacitive reaction takes place not only at the surface but also in the bulk of the film.
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Fig. I-14. The specific capacitance of PANI, WO3 and the composite WP as a function of the
film mass measured through constant current charge–discharge at 1 Ag−1 in 1M H2SO4 [63].
In a reported work by J. Zanget al. [24] a well-aligned cone-shaped nanostructure of PPy was successfully grown on Au substrate by using a simple, one-step, cost-effective, template-free, and anodic deposition method.Furthermore, the 3D, arrayed, nanotubular architecture coated with an ultrathin layer of RuO2 was tailored to construct a supercapacitor. The unique structure
and design not only reduces the diffusion resistance of electrolytes in the electrode material but also enhances its electrochemical activity. The specific capacitance and conductivity of RuO2/PPy by the modification with an ultrathin layer of RuO2 was significantly improved in
comparison to that of the bare PPy, which was verified by CVs and EIS for the two electrodes. The good stability of the RuO2/PPy electrode is very promising for applications in
microsupercapacitor devices.
II-4. Carbon-MO
xComposites:
Owing to their high conductivity, permeability (resulting in high power density) and chemical inertness (long cycle lifetime), single wall carbon nanotube (SWCNT) thin films are promising candidates for active supercapacitor electrode materials [56,76]. In nanocomposite form, metal oxides and conducting polymers could improve electrochemical and mechanical properties of the supercapacitors. Therefore, recent researches has been focused on the exploration of potentiel pseudocapacitive metal oxides have been investigated [44–53,77–79].
R. Yuksel et al. [80] have recently reported on the fabrication of ternary nanocomposite SWCNT/WO3/PANI thin films as supercapacitor electrodes. High electrical conductivity of
SWCNT thin films allowed the possibility of the fabrication of supercapacitors without a separate charge collector and enhanced the properties solely arised by the WO3 and PANI
27
components in a synergistic manner. These fabricated ternary nanocomposite supercapacitor electrodes were found to have a specific capacity of 28.5 mF/cm2 at a current density of 0.13
mA/cm2 therefore they could be promising electrode active materials for electrochemical
energy storage systems if an air-stable conducting polymer can be integrated into the structure. In a study byA Eftekhari et al. [38,39], manganese oxide films were galvanostatically deposited in the presence of a small amount of carbon nanotube (CNT). The resulting film cannot be considered as a CNT based nanocomposite, as no CNT is detected by electron microscopy. However, the manganese oxide electrodeposited delivers an excellent pseudo-capacitive behavior to be used as a superior supercapacitor. The samples showed a specific capacitance of 280 F g-1. As it seems that the capacitance of this electrode is related to the chemisorption of
the alkali cation, an extremely high specific capacitance of 434 F g-1 was achieved in a saturated
medium of Li electrolyte. This high specific capacitance can be attributed to the presence of carbon nanotubes results in the formation of nanostructured films which provide a better ion accessibility. Although the exact mechanism for this phenomenon is still vague, the presence of carbon nanotubes (probably as a solid charge carrier) close to the electrode surface is apparently responsible for a different pathway for the electrodeposition process.
Graphene has also been considered as a promising candidate for a supercapacitor electrode material due to its attractive characteristics such as large surface area, good flexibility, excellent electrical conductivity and wide potential windows. The intrinsic capacitance of single-layer graphene reaches ca. 21 mF cm-2 when the entire surface area is used or accessible. It is also an
excellent substrate to host active nanomaterials for charge storage due to its abundant surface functional groups. Recent advances on graphene/metal oxide composites for electrochemical applications inspired some groups to synthesize T-Nb2O5/graphene nanocomposites. They may
greatly improve the behavior of active T-Nb2O5 nanocrystals and consequently achieve
excellent performance [14].
L Kong et al. [14] presented a simple and one-pot hydrothermal method to decorate Nb2O5
nanoparticles onto the surface of reduced graphene oxide (rGO) sheets. After post-treatment at 700 °C, rGO converted into graphene with improved electric conductivity while the amorphous Nb2O5 nanoparticles recrystallized into T-Nb2O5 nanocrystals. The synergistic effects between
graphene and T-Nb2O5 nanocrystals results in excellent electrochemical capacitive properties
including high capacitance/rate capability and excellent cyclic stability.
Among the metal oxides investigated, NiO with a high theoretical capacitance value of 2573 F g-1, a low cost, a distinct redox reaction, and a controllable morphology have become a popular
